1. Field of the Invention
The present invention discloses a method for measuring microphysical characteristics of natural precipitation using particle image velocimetry, which measures the shape, size, fall velocity, and other physical characteristics of the precipitation particles. The present invention relates to the field of automated weather systems.
2. Description of Prior Art
The detailed knowledge on precipitation microstructures, such as size, shape, fall velocity, and phase of precipitation particles, are crucial to the precipitation remote sensing, terrestrial and satellite radio transmission, tropospheric wave propagation, soil erosion, and other atmospheric sciences and applications.
A number of tools were invented to measure the intensity, and amount of precipitation. At first, the amount of precipitation was recorded by collecting the precipitation in graduated cylinder manually. U.S. Pat. No. 3,943,762 (1976) discloses a tipping bucket rain gauge, which utilizes two oscillating reservoirs to collect rainwater and measure the rate and amount of rain automatically, but it can underestimate the rain intensity of a blowing rainfall and the duration of a light rainfall. According to the system disclosed in U.S. Pat. No. 6,038,920 (2000), a precipitation measuring system is used for electronically measuring the amounts of rain, snow, hail or the like, with the use of a load cell as a weight sensor. These instruments are now widely used as operational instruments, but they are not able to measure the microphysical features of precipitation.
Present weather observing system (U.S. Pat. No. 4,613,938, 1986) includes a radiation source and detector which can detect the scattered radiation from suspended or precipitating particles within a sample volume, it can obtain the intensity and types of precipitation. An optical precipitation gauge (U.S. Pat. No. 4,754,149, 1988) employs an optical transmitter and receiver to detect scintillations produced by particle movement in the light beam. A laser precipitation sensor (U.S. Pat. No. 5,298,750, 1994) can identify and record the precipitation type, size and intensity by detecting the beam of laser light scattered by particles. An optical and acoustic weather identification system (U.S. Pat. No. 5,528,224, 1996) employs a light beam source that transmits a partial coherent beam to detect precipitation, and distinguish snow, sleet, and hail from rain by an acoustic receiver. A weather identifier system (U.S. Pat. No. 6,914,674, 2005) employs two photosensitive receivers to detect the scintillation and forward scattering of light from a partially coherent light beam source, which can identify the rain, snow, and drizzle.
According to the system disclosed in U.S. Pat. No. 7,249,502, an impact disdrometer infers the size of the individual drops from the measured impact kinetic of the drops through an empirical nonlinear relationship between fall velocity and drop diameter in still air, but it cannot measure the shape and velocity of raindrops. OTT PARSIVEL disdrometer can measure the horizontal size of particles using the decrease of light signal by extinction; the vertical size and fall velocity of particles are estimated by the empirical assumption of raindrops' shape, but the difference between assumption and real rainfall can cause distortion of drop size distribution and velocity distribution. Also, this device cannot measure the particles' shape.
Both the 2D Video Disdrometer (2DVD) and Hydrometeor Velocity Size Disdrometer (HVSD) have two line-scan cameras; therefore the vertical velocity of each particle can be determined according to the distance between the two light sheets and its traveling time. 2DVD can measure three-dimensional raindrop shape information, while HVSD can only measure two-dimensional raindrop shape information. Since the raindrop image is acquired in sequential planar sections as it falls through the light sheet, the horizontal motion of the droplet in the presence of horizontal winds causes a distorted image of the raindrops. While such distortion may be corrected, errors in the drop shape measurement are inevitable. Snow Video Imager (SVI) can measure the shapes and size distributions of snowflakes using a CCD image sensor illuminated by a halogen flood lamp, but it cannot measure the fall velocity of snowflakes/raindrops.
Accordingly, there remains the need for measuring the size, shape, fall velocity of precipitation particles simultaneously.